Provide token to show number of items in next page in button text

Looking good, thanks for the test. Review as follows.

1. +++ b/src/Plugin/views/pager/InfiniteScroll.php
   @@ -23,6 +23,27 @@ class InfiniteScroll extends SqlBase {
   +    if (!empty($text) && strpos($text, '@') !== FALSE) {
   

   Is @token_name the approach the rest of views uses? I think they faked twig and started using {{ foo }} for their tokens? There might be some views methods that deal with this.

2. +++ b/src/Plugin/views/pager/InfiniteScroll.php
   @@ -23,6 +23,27 @@ class InfiniteScroll extends SqlBase {
   +      $items_per_page = (int) $this->view->getItemsPerPage();
   +      $total = (int) $this->getTotalItems();
   +      $current_page = (int) $this->getCurrentPage() + 1;
   +
   +      // Default to the pager amount.
   +      $next_page_count = $items_per_page;
   +      // Calculate the remaining items if we are at the 2nd to last page.
   +      if ($current_page >= ceil($total / $items_per_page) - 1) {
   +        $next_page_count = $total - ($current_page * $items_per_page);
   +      }
   +
   

   Lets move everything that calculates the number of items left into a getNumberItemsLeft method. Almost a good candidate for a unit test.

3. +++ b/src/Plugin/views/pager/InfiniteScroll.php
   @@ -23,6 +23,27 @@ class InfiniteScroll extends SqlBase {
   +      $replacements = [
   +        '@next_page_count' => $next_page_count,
   +      ];
   +      $text = str_replace(array_keys($replacements), array_values($replacements), $text);
   

   strtr will do key/value replacements without array_keys/array_values.

4. +++ b/src/Plugin/views/pager/InfiniteScroll.php
   @@ -72,6 +93,14 @@ class InfiniteScroll extends SqlBase {
   +    $item_list = [
   +      '#theme' => 'item_list',
   +      '#items' => [
   +        '@next_page_count -- the next page record count',
   +      ],
   +    ];
   +
   +    $list = $this->getRenderer()->render($item_list);
   
   @@ -84,6 +113,7 @@ class InfiniteScroll extends SqlBase {
   +        '#description' => $this->t('The following tokens are supported:') . $list,
   

   We can probably use #prefix/#suffix on the list to get a description in there, instead of rendering early.

5. +++ b/tests/src/FunctionalJavascript/InfiniteScrollTest.php
   @@ -74,6 +74,16 @@ class InfiniteScrollTest extends JavascriptTestBase {
   +    // Test @next_page_count token.
   +    $this->createView('next-page-count', [
   +      'button_text' => 'Load @next_page_count more',
   +      'automatically_load_content' => FALSE,
   +    ], 6);
   +    $this->drupalGet('next-page-count');
   +    $this->getSession()->getPage()->clickLink('Load 5 more');
   +    $this->getSession()->wait(static::ajaxWaitDelay);
   +    $this->assertTotalNodes(11);
   

   <3

Straight reroll of #4 as it no longer applied.

My use-case is to have a string as: Load More @next_page_count more tickets (@total tickets total)
So I'm adding @total token as well.

This dropped off the radar. Looks good to go for me.